Many .beta.-lactam containing antibiotics have an acylamino side-chain of the formula ##STR2## The symbol "R.sub.a ", as used here and throughout the specification, can be carboxyalkyl (preferably carboxymethyl or 1-carboxy-1-methylethyl). Exemplary antibiotics include aztreonam and ceftazidime. Many processes are described in the prior art for preparing compounds of this type. One such process (see, for example, U.S. Pat. No. 4,443,374) comprises the initial preparation of a compound having the formula ##STR3## (here and throughout the specification the symbol "A" can be the nucleus of a .beta.-lactam containing antibiotic and the symbol "R" can be an amino protecting group), followed by reaction of that compound with an aminoxy derivative of the formula EQU H.sub.2 N--O--R.sub.a,
followed by deprotection of the resulting compound. This reaction yields a product which exists as the syn or anti isomer or as a mixture of isomers. In the antibiotic art it has been found that the syn isomer exhibits greater activity than the anti isomer.